feat: Added usage of storage backend for vault-plugin-secrets-mock

[siredmar]

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This PR adds the usage of the storage backend to make changes persistent to the example plugin vault-plugin-secrets-mock.

Why: It was quite unclear to me how a custom plugin could persist its data. Searching for the required methods in the many built-in plugins was too extensive for such a simple answer. Therefore, this improved example should provide more help to newcomers like me.

Signed-off-by: Armin Schlegel [email protected]

[hashicorp-cla]

All committers have signed the CLA.

[siredmar]

@yhyakuna would you mind reviewing this?